1. Field of the Invention:
The present invention relates to a gamma-ray compensated ionization chamber. More particularly, it relates to a gamma-ray compensated ionization chamber in which the variation of the compensating characteristics caused by the variation of the external temperature and the variation of the gamma-ray energy is reduced.
2. Description of the Prior Arts:
The ionization chamber shown in FIG. 1 has been known.
In FIG. 1, a cylindrical signal electrode (2) is coaxially placed outside of the cylindrical compensation electrode (1). A high voltage cylindrical electrode (3) is coaxially placed outside of the signal electrode (2). The high voltage electrode (3), the compensation electrode (1) and the signal electrode (2) are combined to form multiplex electrodes (4). The electrodes (1), (2), (3) are held in a metal casing being an airtight container. A neutron sensitive substance (6) such as boron and uranium (the reference (6a) is in the side of the high voltage electrode; and (6b) is in the side of the signal electrode (2)) is coated on the inner surface of the high voltage electrode (3) and on the external surface of the signal electrode (2). The casing (5) is electrically insulated from the high voltage electrode (3) through a ring insulator (7a) which holds the electrodes in place. The high voltage electrode (3) is electrically insulated from the signal electrode (2) through the ring insulator (7b). The signal electrode (2) is electrically insulated from the compensation electrode (1) through the ring insulator (7c). They are coaxially held. The insulators (7a), (7b), (7c) are designated by the reference numeral (7). The electrodes (1), (2), (3) are respectively and electrically connected through the terminals (8) placed outside of the casing (5). The terminal (8) comprises a compensation electrode output terminal (8a), the signal electrode output terminal (8b) and the high voltage electrode output terminal (8c). The compensation electrode (1) is electrically connected through a lead wire (9a) to the compensation electrode output terminal (8a). The signal electrode (2) is electrically connected through the lead wire (9b) to the signal electrode output terminal (8b). The high voltage electrode (3) is electrically connected through the lead wire (9c) to the high voltage electrode output terminal (8c). The lead wires (9a), (9b), (9c) are designated by the reference numeral (9). An ionizable gas (10) for ionizing between the electrodes (1), (2), (3) is sealed inside of the casing. A small hole (H) is formed on each of the end surfaces of the cylindrical electrodes (1), (2), (3) at the reverse side to the terminals (8) and the ionizable gas (10) is freely moved through the small holes (H). Each end surface having each small hole (H) is formed in one body with the cylindrical part of each electrode.
The condition in which the radioactive ray radiates into the ionization chamber having said structure will be illustrated.
The ionizable gas (10) between the compensation electrode (1) and the signal electrode (2) is ionized by the radiation of the radioactive ray thereby forming secondary electrons emitted from the surface of the electrode by gamma-ray. Moreover, a part of the gamma-ray directly actuates the ionizable gas to cause ionization. When negative voltage is applied to the compensation electrode (1), the ionization I.sub..gamma. is passed from the signal electrode (2) to the compensation electrode (1). The ionization is also caused by the gamma-ray between the signal electrode (2) and the high voltage electrode (3) the same as between the signal electrode (2) and the compensation electrode (1). The neutron sensitive substance (6a), (6b) reacts with neutrons whereby the ionizable gas is ionized by the resulting charged particles at high velocity. Therefore, when the positive voltage is applied to the high voltage electrode (3), the neutron current I.sub.n being proportional to the intensity of neutron and gamma-ray ionization current I.sub..gamma., being proportional to the intensity of the gamma-ray, are passed from the high voltage electrode (3) to the signal electrode (2). Thus, the signal current I.sub.s : I.sub.s =I.sub.n +I.sub..gamma. '-I.sub..gamma. is passed to the signal electrode (2).
The currents I.sub..gamma. and I.sub..gamma. ' caused by the gamma-ray are proportional to the surface area of the electrodes and the number of the molecules of the ionizable gas between the electrodes. The condition of almost I.sub..gamma. '=I.sub..gamma. can be given by selecting the diameter of the electrodes. In such case, the signal current I.sub.s is proportional to the neutron beam as the signal current I.sub.s =neutron current I.sub.n.
As described above, the purpose of the ionization chamber is to offset (cancel) the gamma-ray current and to obtain only neutron current I.sub.n.
In this type of ionization chamber, it takes a long time for heat conduction into inside part depending upon the variation of the external temperature, to cause the temperature difference between the inner part and the external part. The ratio of spaces between the electrodes varies depending upon the difference of the thermal expansions to cause the difference between the gamma-ray currents to be I.sub..gamma. '.noteq.I.sub..gamma.. Therefore, the error results since the signal current I.sub.s varies from the neutron current I.sub.n for the gamma-ray currents I.sub..gamma., I.sub..gamma. '.
When the gamma-ray having different energy is radiated, the gamma-ray shielding coefficient by the electrode varies depending upon the energy i.e. the wavelength. Therefore, the intensity of the gamma-ray reaching the inside of the ionization chamber varies depending upon the energy of the gamma-ray. The gamma-ray ionization current between the electrodes varies depending upon the wavelength to cause a change of the gamma-ray compensation, disadvantageously.